1. Field of the Invention
This invention relates to a display device and in particular to a plasma display device used for displaying information such as a television receiver and a computer display, and to an optical filter for a display device incorporated into the display device.
2. Description of the Related Art
In general, an optical filter of which optical transmittance is adjusted to a prescribed value is incorporated into a display device so as to improve a contrast in bright locations (hereinafter referred to as a bright-location contrast). FIG. 6 shows the function of the optical filter. As shown in FIG. 6, in the display device, the optical filter 11 is provided on the front side, i.e., the display-surface side, of a display panel 10. Light 12 emitted from the display panel 10 passes through the optical filter 11 and exits to the exterior of the display device. On the other hand, a part of the ambient light 13 incident to the optical filter 11 of the display device from the external environment is reflected by the front surface of the optical filter 11, and then becomes a reflected light 14, and the rest of the ambient light 13 passes through the optical filter 11, reflects off the front surface of the display panel 10, passes through the optical filter 11 again, and then becomes a reflected light 15.
As a result, the reflected lights 14 and 15 are mixed with the light 12 emitted from the display panel 10, so that the bright-location contrast of the display device decreases. In actuality, the reflected light 14 reflected at the front surface of the optical filter 11 is extremely weak as compared with the reflected light 15 reflected at the front surface of the display panel 10, so that the bright-location contrast of the display device is greatly affected by the transmittance of the optical filter 11. Assuming the transmittance of the optical filter 11 as t, an intensity of the light 12 after passing through the optical filter 11 is expressed as an intensity of the light 12 emitted from the display panel 10 multiplied by t, whereas the intensity of the reflected light 15 which has passed through the optical filter 11 twice is expressed as an intensity of the ambient light 13 multiplied by t2. Hence if the reflected light 14 is neglected, the bright-location contrast, which is defined by an intensity of light emitted from display panel divided by an intensity of reflected light from outside, becomes t/t2=1/t. For example, if the transmittance t of the optical filter 11 is 60%, then the bright-location contrast becomes 1/t=1/0.6≈1.7. Accordingly, provision of an optical filter 11 in the front-surface side of the display panel 10 improves the bright-location contrast.
A plasma display panel (hereafter referred to as a PDP), which is recently getting attention due to its capability to providing a large screen and flat shape, has two transparent substrates arranged parallel to each other, and a plurality of display cells provided between the two transparent substrates. The display cells are filled with a rare gas such as helium, neon and xenon. Discharge of the rare gas generates ultraviolet rays, and these ultraviolet rays excite a fluorescent material to cause light emission. Accordingly, images are displayed. However, because the discharge emission wavelength of neon exists in the visible range (in the red-orange range), there is a problem that the emission of neon deteriorates color repeatability of the plasma display.
FIG. 7 is a graph showing an example of the spectrum of a conventional optical filter in which an abscissa denotes wavelength of light and an ordinate denotes optical transmittance. In FIG. 7, the spectrum 21 shows a spectrum of a conventional optical filter. As shown in FIG. 7, the optical transmittance of this optical filter is suppressed to approximately 65% at maximum. As a result, if this optical filter is incorporated into a display device, the bright-location contrast of the display device is improved. The spectrum of this optical filter has regions where the transmittance is lower as compared with other regions, such as 21a, 21b and 21c. The region 21a is a discharge emission region of neon, the region 21b is a near-infrared region, and the region 21c is the near-ultraviolet region. Reduction of the transmittance in the region 21a allows absorbing the emission of neon, and thus the color repeatability in image displaying can be enhanced. On the other hand, reduction of the transmittance in the regions 21b and 21c allows absorbing excess light. Thus, selectively establishing the wavelength region having low transmittance enables a color filter to adjust the hues of the display device.
In Japanese Patent Kokai No. 2000-105541, pages 3 through 6, an optical filter with an object of enhancing the contrast and color repeatability of the display device is disclosed in which a ray transmittance at wavelength of 450 nm is greater than the ray transmittance at wavelength of 525 nm, and the ray transmittance at wavelength of 525 nm is greater than the ray transmittance at wavelength of 630 nm. It is also disclosed in this optical filter that the ray transmittance at wavelength of 580 nm is 60% or less. Further, in Japanese Patent Kokai No. 2003-157017, pages 3 through 5 and FIG. 4, an optical filter with an object of enhancing the bright-location contrast of the display device is disclosed which has an absorption band near 490 nm (in a region between blue and green), and the transmittance at 490 nm is lower than the transmittance of white light of the PDP.
The above-described conventional technology has the following problems. In a conventional optical filter, the transmittance in a wavelength region of the light emission of the display device is set higher than the transmittances in other wavelength regions in order to maintain high brightness of the display device. But currently, display devices are often installed in indoor environments where fluorescent lamps are often used as indoor illuminations so that the ambient light is mostly the light from the fluorescent lamps. As a result, the most outstanding factor that decreases the bright-location contrast is the reflected light from the florescent lamps. However, since the emission spectrum of the fluorescent lamp often has high intensity in the same region as the emission wavelength of the display device, the above-described conventional optical filters cannot sufficiently reduce the intensity of the reflected light caused by the fluorescent lamps.